1. Field of the Invention
This invention relates in general to a method and apparatus for producing multi-color displays, and, more particularly, to a field sequential color head-up display system.
2. Description of the Related Art
It is well known in the prior art that an apparently monochromatic source, such as a cathode ray tube (CRT), can provide a multi-color display. From the earliest experiments in color television which used a rotating color wheel together with a CRT, it was possible to place images on the screen in synchronism with the color wheel so that a number of colors could be visualized. This phenomenon was as much a product of the persistence of the displayed image in the human eye as it was of the synchronization of certain displayed images with the concurrent presentation of a filter of preselected color.
The subsequent development of the shadow mask tube and the use of triads of monochromatic phosphors were deemed preferable improvements over the electromechanical color wheel and, as a result, the color television industry abandoned the electromechanical approach to color displays. It has been found in the past that a polychromatic source of light which appears to be monochromatic (from the mixing of colors) can provide images in both the principal colors and in colors which result from a mixture of the principal colors.
As early approach to an electronic field sequential color television system is described in U.S. Pat. No. 3,781,465 to Ernstoff et al. That patent shows a monochromatic cathode ray tube with a tri-color liquid crystal assembly that included red, blue and green cells. Electronic circuitry switches the three cells sequentially in a preselected order to provide a series of single color images in rapid succession that, due to image persistence in the human eye, give the appearance of being in full color.
A different approach in described in U.S. Pat. No. 4,003,081 to Hilsum et al. The approach in this reference is to select the cathode ray tube as a source of light that produces an image in at least two colors that are subsequently combined by an electrically controlled filter element. The active component of the filter is a liquid crystal material capable of separating an transmitting different colors through the filter depending upon the energizing control signals applied to control the filter element. In a preferred embodiment, video images from a first data source, such as radar, were to appear in one color while video images from a second data source, such as a computer, were presented in a second color. Alternative embodiments include a field sequential color display which includes a third filter so that a three color combination is possible.
U.S. Pat. No. 4,385,806 to Fergason, teaches a liquid crystal light shutter device used in combination with retarding wave plates to compensate for retardation in the light shutter device while it is operating under a control bias. By utilizing a control bias, the reference suggests that the operation of the liquid crystal light shutter device is speeded up, and that the retardation plates are compensated to account for the normal birefringence effects of the liquid crystal cells comprising the light shutter device.
In U.S. Pat. No. 4,436,376 to Fergason, a pair of liquid crystal cells are operated similar to a "push-pull" amplifier, in that each liquid crystal cell functions to impart its own phase shift to a passing optical beam. The application of a control bias to electrodes in each liquid crystal cell, aligns a preponderance of the liquid crystals in each of the cells, except for those liquid crystal layers immediately adjacent the electrodes. The reference states that application of incremental electrical signals across the conducting surfaces of the liquid crystal cell achieves a rapid on-off cycle for the cell. The cells thus taught are utilized as part of a communications link to modulate a light beam by means of applied electrical signals to the cells. Control signals that are 180.degree. out of phase are applied to the two liquid crystal cells for "push-pull" operation.
The general idea of using a "black and white" cathode ray tube in conjunction with liquid crystal cells and color polarizers is described by Brinson et al in IBM Technical Disclosure Bulletin, Vol. 22, No. 5 of October, 1979. In order to provide a full three color capability, a first liquid crystal cell is followed by a first color polarizer and a second liquid crystal cell is followed by a second color polarizer. Essentially "white" light is transmitted through a linear polarizer and, depending upon the state of the first cell, either cyan or red is passed to the second cell. Depending on the state of this second cell, either blue or yellow is passed to the observer. The net output of the combination to an observer is then either blue, red, green, or black at any instant of time. During operation, the net colors would be "mixed" by having images persist through more than one output color phase to provide a substantially full palette of colors to the observer.
A slightly different approach was disclosed in U.S. Pat. No. 4,328,493 to Shanks et al. A cathode ray tube which emits at least two different colors is combined with first and second color selective polarizers, a liquid crystal cell and a neutral linear polarizer. The liquid crystal cell in one condition rotates the plane of applied polarized light and in a second condition transmits the light without rotation. The cell is the switched in synchronism with the presentation of the images that are to be seen in color. Because the cells cannot be switched between states in the time available during television transmissions, only one half of the cell is switched at a time and the electrodes are driven accordingly.
In the published U.K. patent application of Bos et al, GB 2 139 778 A, published Nov. 14, 1984, corresponding to a U.S. application Ser. No. 493,106, filed May 9, 1983, a field sequential color system is disclosed. In this reference, a liquid crystal cell functions as a variable optical retarder in a polarizing system and includes pleochroic filters which selectively transmit a first or a second color, depending upon the polarization of the light. A color sensitive polarizing means are placed in front of a cathode ray tube which is capable of emitting light of at least two colors. A first absorption axis passes linearly polarized light of the first color and a second absorption axis passes linearly polarized light of the second color. The liquid crystal cell is followed by a linear polarizer.
When the liquid crystal cell is driven by a first signal, it provides a half wave retardation to applied light. When driven by a second signal, substantially no retardation is experienced by the impinging light. With substantially no retardation of light, only light of one of the two colors can pass through the linear polarizer. With half wave retardation, only light of the other of the two colors can pass through the polarizer.
A specially designed liquid crystal cell functions as the variable optical retarder. The preferred cell is a nematic liquid crystal cell designed to be disinclination--free and to switched in a "bounce-free" manner as it is switched between its two states which alter the orientation of the surface non-contacting directors of the liquid crystal material in the cell.
There have been a number of attempts to modify the above described color technology for application to image generation and display systems, such as for use in head-up display systems.
In general, many vehicles, such as aircraft, have been fitted with optical systems that project an image which to the vehicle operator appears to be a part of the exterior scene that is viewed by the operator from the interior of the vehicle.
The earliest applications of this concept were gun sight aiming systems in which a target reticle was provided the vehicle operator for use in aligning the vehicle with a target thereby simultaneously aiming the vehicle's weapons systems at the selected target.
In recent years, with the advent of more sophisticated computer systems and symbol generation techniques, head up display systems have been used to place additional information in the field of view of the vehicle operator or pilot so that it becomes less necessary for the operator to look away from the exterior scene normally visible through the windscreen in order to glance at the instrument panel. Providing the head up display enables the pilot to pay more attention to the situation around him and avoids the need to refocus his eyes which would otherwise be required if the instrument panel were to be read.
Accordingly, in recent years, head-up display systems have become a vital part of the cockpit of fighter and attack aircraft, and, to a lesser extent, in commercial aircraft. Special head-up displays have been employed in some spacecraft and are in use in the space shuttle.
Prior art head-up displays, such as that shown in U.S. Pat. No. 4,001,499 to Dowell, include electronic image generating means which create a visible image on a cathode ray tube. Because of the crowded conditions in the instrument panel, and the generally elongated shape of the cathode ray tube, the display assembly is usually packaged in a substantially rectangular box that is placed with its long axis generally parallel the axis of the aircraft. The image produced on the face of the cathode ray tube will then be somewhat below the line of the pilot's normal view through the windscreen.
The image thus generated by this display system is then projected vertically to a semi-reflective combiner screen through which the pilot views the windscreen and the scene exterior thereto. To the pilot's eye the exterior scene then appears with the generated images superimposed thereon.
In spite of all the prior art known to the applicant and that discussed above, no prior art reference discloses a field sequential color head up display system that is sufficiently compact in size and flexible enough to be controlled or modified through programmable software. Similarly, no prior art reference teaches a multi-color cathode ray tube having sufficient brightness and resolution to provide a practical image generation source of light for use in field sequential color head up display systems.
The present invention discloses a practical and successful field sequential color head up display system that remedies the shortcomings of the prior art.